Biogas Pipe Network

[wjog4]

I originally posted this in the gas distribution engineering forum, but I'm not sure how much attention I would receive there, so I thought maybe I should post here as well for more opinions.

I'm currently working on a project to distribute biogas from multiple digesters from multiple locations to a facility. At this time I cannot give specifics due to confidentiality, but I have some concerns with the work done so far on this project, and also some work I found on a similar project that has yet to be constructed. Both of these projects attempt to transport biogas with blowers at obviously low pressures through a pipe network with line sizes ranging from 4 to 10 inches through miles of pipe (approx. 34 miles of total pipe for the network I'm looking at). The flows range from 48 to 465 scfm from the various digesters, for a maximum of 2,406 scfm (for about a 10,000 ft. run) at the largest point in the system.

I have obvious concerns with trying to transport biogas at these pressures, over these distances. In my short (3 year) engineering career I've never encountered an attempt to transport a gas a such low pressures across such a distance of pipe. Not only would I be concerned if it were just one pipe segment, but a network is even more concerning. I took the even the shortest runs of pipe with the lowest flows and calculated pressure drops higher than the inlet pressure (I also used TLV.com with a MW of 21.8, a viscosity of 0.01297 cP, and temperature of 70 deg F). To make matters worse, the reference project had a "blower calculation" spreadsheet that was attempting to determine the size of blowers and their required discharge pressures. This project which deals with similar flows, pipe sizes, and pressures was using the Weymouth formula! As far as I know this formula is only applicable for natural gas piping, at large pipe diameters, large flows, and high pressures (according to Crane TP 410, my Mechanical PE book, and my reading of "Transactions of the Society of Mechanical Engineers Volume 34"). I imagine maybe using the isothermal equation may be of more value in this scenario, but I'm not sure how to apply all of this to a biogas pipe network.

I also realized that each junction pressure must match the line pressure it's feeding into, and realized that maybe the best approach for a network such as this is to determine a required delivery pressure, and calculate backwards to each junction, and subsequently back to each blower (or compressor since I believe they are needed)?

A final question I have is, is it even economically viable to try to transport biogas along a large pipe network such as this? I'm thinking once I figure out the HP of the compressors required that compressing the gas to a pressure transportable through a large system such as this, it will leave me with negligible net work that the biogas will provide at the facility it is being transported to due to biogas' low heating value.

Any help would be much appreciated, I'm sorry if I'm scattered about in my thoughts, feel free to ask for any clarifications.

 

[BigInch]

Low pressure gas pipeline gathering systems are pretty common in the gas field production systems. I used to run one in south Texas that had 1600 miles of 2-20" diameters which could deliver nearly 1 Billion SCFD from 860 wells, but the main lines were much higher pressure and I had several large compressor stations and many 1500 HP field compressors at critical points within the gas fields near Laredo.

Low pressure systems are not an efficient means of transporting large volumes of gas for very long distances, where you must use pressures of 800-1400 psig and numerous, large compressor stations, but they can and do work well enough for smaller field gathering systems, say 25-50 miles across.

You are correct that the Weymouth formula is NOT APPLICABLE in this situation. Shocking, but it wouldn't be the first time.

You will need to do exactly as you say. Determine what outlet pressure is needed from your system and work upstream into the tree of pipe connecting the source wells, putting in blowers or small compressors whenever the upstream pressure reaches near to the allowable pressure of your pipe.

Try this flow calculator

http://www.druckverlust.de/Online-Rechner/dp.php

I got an 8" pipe diameter for 10,000 ft flowing 2500 scfm with about a 50 psi loss, so your system sounds like it is in the right range.

Maybe you don't have this little gem. The US Army Corps of Engineers Manual for Landfill Gas Collection Systems. I'm sure you will want to download it. Tax money already paid for it.

http://www.publications.usace.army....als/EM_200-1-22.pdf?ver=2013-09-05-152155-217

BTW The Spitzglass equation should be used for very low pressure gas piping (pressure drops measured by inches of water column).

http://www.engineeringtoolbox.com/natural-gas-pipe-calculator-d_1042.html

 

[LittleInch]

wjog4,

Sounds like an interesting project.

you also need to think about the CAPEX costs in all this as well.

There is always a balnace between high pressure / small diameter versus lower pressure bigger diameter in terms of CAPEX and OPEX.

To transport this gas on a continuous basis you will need something better than "blowers" unless you have a tunnel system of 1.5m diameter....

There will likely be a step change in costs at something like 10 bar and 16 bar where you can't use PE pipe anymore ( available on long reels at small sizes) and cheap and easy to install. After that you're looking at steel or RTP.

Maybe some of your low producers you install a small PD pump and pump into a cylinder or bank of cylinders and then transport it by truck to a location where it can feed back into the system?

It all depends on the details, locations, sizes and flows. Some might be able to be solar powered?

A network analysis can become complex, but yes I agree with BI, start and the end point and work backwards to find your critical line

Is it viable - I don't know - that's why engineers like you get paid to find out

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BigInch]

For zones with really low pressure wells, some have suggested that a blower at the well and "gas bag" pipelines be used until you get to the first compressor. A lot like plastic rubbish bags glued together end after end. That will get you large diameters, until the first big wind, or ice/snow comes along. Tropical climates only???

 

[LittleInch]

Or just go and modify your car to run on the stuff....

http://www.lowtechmagazine.com/2011/11/gas-bag-vehicles.html

This really happened.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[wjog4]

BigInch and LittleInch, thank you both for your help with this so far.

LittleInch, I agree that this project is very interesting. I can tell you that the gas is coming from multiple dairy lagoon digesters at essentially atmospheric pressure, so I think this project is very unique and has many interesting problems to tackle.

BigInch, from your first response I am on the one hand thinking that I may be partially correct in that blowers alone will not be able to deliver biogas through the entire system. However your comment about low pressure pipelines working for systems 25-50 miles across, makes me think that it can be done, but can you clarify the range of pressures that you are defining as low pressure? I think some flows are low enough to be piped at low pressures (6 psi) provided the pipe diameter is larger than what I'm seeing. This brings me to LittleInches point about CAPEX and the balance between low pressure, large pipe, and higher pressure, small diameter pipe. I was hoping to use (just as LittleInch said) spooled pipe (I only know of fiberspar) for the areas where possible, but I realize that I will need to look at the OPEX of compressing the gas to a high enough pressure to be transported down smaller lines and if it is worth the energy expended in compression and pipe losses to save in capital and construction.

I think I will be spending any downtime I have over this long weekend digging into this more and doing just as we discussed, working from the point of delivery backwards. You both have given me a lot to think about and look in to so thank you for that.

Edit: LittleInch, that photo is great, I think you're on to something. Oddly enough, I'm also working on a CNG station currently, so I think I have two applications for that car.

 

[LittleInch]

wjog 4

At the pressures and sizes you're looking at you need to look at reeled PE pipe. Anything less than 180mm OD can normally be supplied reeled. The smaller the pipe the longer you get on one reel.

You probably only need the stuff rated for 6 bar so should be quite cheap. The flexible stuff you refer to is really for high pressure wells and injection systems and costs a lot more than PE.

You might need a big bag to collect the stuff and then turn the compressor on every now and then to empty it. The volumes will be much bigger at low pressure so you'll find it difficult to get it down in size otherwise or use a gasometer type arrangement ( works on a few inches water column pressure)

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BigInch]

If I remember correctly the COE manual has pressure drop calculations within. Just follow those if they are there, otherwise come back here and ask again and we'll figure out which equations you need to be using.

There's quite a few of these municipal gas collection systems around, so the economics must work somehow, but you need to be using PE "Yellow gas pipe" as LI says. Fiberspar is way too expensive and some of that reinforced resin stuff is more expensive than steel pipe, and even though transport and handling is easier, it doesn't work economically until you can use all it's high pressure capacity to move a lot more gas than the 3.6 MMSCFD you've got there.

Depends on flow and pressure at your well heads as to if you need, or can use blowers. Ideally you'll have enough pressure at each well head that you can use a compressor. If you don't have enough flow from one well to get into compressor sizes, you'll probably want to tie in several wells, maybe with blowers and low pressure piping to get you to a junction where you have enough flow to warrant using a compressor and switch to higher pressure pipe after that. See what compressors can offer the lowest flow and inlet pressure characteristics at a good price and use that as your base minimum flow to decide where the low pressure / high pressure break nodes might be located. Then run your low pressure gathering system from wells to those nodes, then use higher pressure piping after that (50 - 100 psi maybe, let your long runs decide what that pressure must be by backcalculating the compressor discharge pressure from your known minimum system discharge pressure) and flow rate.

(W)->(B>)--LP-------|
(W)---LP--+(C>)----HP--------> Min System Discharge Press
(W)--LP-|